Image intensifier tubes are utilized to enhance night time vision without using additional light. These devices have both military and industrial applications. The U.S. military uses image intensifier tubes for viewing and aiming at targets at night that otherwise would not be visible. In addition, image intensifier tubes are used by aviators to enhance night time vision, for providing night vision to people who suffer from night blindness (retinitis pigmentosa) and for photographing astronomical bodies.
Generally, an image intensifier tube includes three main components. These components include a photocathode, a phosphor screen (anode) and a microchannel plate (MCP) disposed between the photocathode and anode. The photocathode is a photoemissive wafer that is extremely sensitive to low radiation levels of light in the 580-900 nm spectral range. When electromagnetic radiation impinges on the photocathode, the photocathode emits electrons in response.
The MCP is a relatively thin glass plate having input and output planes and an array of microscopic holes through it. An electron impinging on the MCP results in the emission of a number of secondary electrons which, in turn, cause the emission of more secondary electrons. Therefore, each microscopic hole acts as a channel type secondary emission electron multiplier having an electron gain of approximately several hundred. The electron gain is primarily controlled by a potential difference between the input and output planes of the MCP. Consequently, the MCP increases the density of electron emission.
The anode includes an output fiber optic window and a phosphor screen which is formed on a surface of the window. Emitted electrons are accelerated towards the phosphor screen by maintaining the phosphor screen at a higher positive potential than the MCP. The phosphor screen converts the electron emission into an image which is visible to an operator.
A type of image intensifier tube known in the prior art is a GEN III Image Intensifier Tube, which is manufactured by ITT Electro Optical Products Division in Roanoke, Virginia. This type of tube utilizes a :photocathode manufactured from gallium arsenide. Such photocathodes are susceptible to being bombarded by positive ions from the MCP, thus degrading the performance of the photocathode. A method utilized to inhibit positive ion bombardment of the photocathode includes coating the MCP with an insulating film of aluminum oxide. This film acts as an ion barrier thus protecting the photocathode and maintaining its performance capabilities.
Resolution of an image intensifier tube is based upon its ability to resolve line pairs. When exposed to a bright source, however, the photocathode emits an increased number of electrons. Due to MCP gain, the increase in electrons generally causes some channels in the MCP to become saturated. This saturation degrades the resolution of the image intensifier tube. As the source becomes brighter, more electrons are emitted by the photocathode, causing more channels in the MCP to become saturated and a further degradation of resolution.
A method utilized to improve resolution of an image during high light conditions employs bright source protection circuits in the power supply. Generally, these circuits lower the potential supplied to the photocathode in response to high light conditions, thus reducing the photo current of the photocathode and the energy of the emitted electrons. However, if the voltage is lowered such that the emitted electrons from the photocathode do not have sufficient energy to penetrate the insulating film, they will begin to accumulate on the film. Consequently, the photocathode voltage is essentially lowered to the secondary emission crossover voltage of the insulating film. This crossover voltage, commonly known as a tube clamp voltage, causes the image produced by the tube to fade out as the insulating film accumulates a negative charge.
To prevent the image from fading out, the bright source protection circuit clamps the photocathode voltage above the tube clamp voltage. This is achieved by maintaining the photocathode at a power supply clamp voltage. Consequently, the image intensifier has the capability of providing acceptable resolution under severe high light conditions.
A predetermined amount of resolution degradation is acceptable in an image intensifier tube. During a high light resolution test, the photocathode is exposed to a relatively high light (i.e. 20 foot-candles) which includes a resolution pattern. Consequently, the power supply senses a high photocathode current and goes into bright source protection mode by lowering the photocathode voltage and the MCP operating voltage. Both of these changes reduce the flow of electrons through the insulating film, causing the electrons to accumulate on the film. Consequently, this causes a degradation in resolution. However, as long as the power supply clamp voltage is kept above the tube clamp voltage, the resolution pattern remains acceptable (i.e. greater than or equal to 5 line pairs per mm) during the high light resolution test.
The power supply clamp voltage is selected between a range of 28-44 volts. However, the tube clamp voltage is not always known since it is determined by the secondary emission characteristics of the insulating film. Typically, the tube clamp voltage will vary from 15 to 30 volts. Moreover, the tube clamp voltage is dependent upon the insulating film thickness, surface conductivity, bulk conductivity, the manufacturing process utilized, and the material used to fabricate the film.
The thickness of the insulating film is an important element in an image intensifier tube's performance. Typically, the film is only 30 to 50 angstroms thick and is extended over a 10 micron diameter opening. This is equivalent to stretching a 0.0005 inch sheet of material such as MYLAR over a 1 inch diameter hole. Consequently, the thickness of the insulating film is dependent on the manufacturing process and is difficult to control. If the resulting film thickness is sufficiently thin, it may not endure normal manufacturing processes, including vacuum baking. This would result in a degradation of the photocathode performance since it would not be protected from positive ion bombardment. If the film is too thick, it will impede the transmission of electrons emitted from the photocathode and reduce the signal to noise ratio.
Therefore, secondary emission characteristics of the insulating film and tube clamp voltage varies for each image intensifier tube. Consequently, the problem of image intensifier tube resolution is exacerbated if the voltage difference between the substantially constant power supply clamp voltage and the tube clamp voltage is sufficient to cause electrons to accumulate on the insulating film.
In vacuum tubes utilizing a glass or ceramic vacuum envelope, the envelope wall generally includes the uncontrolled insulating film. In such tubes, the secondary emission characteristics on the surface can be controlled by a high resistance coating of chrome oxide or iron oxide. Such coatings provide acceptable results when used on the wall of the envelope. However, in an image intensifier tube such as the GEN III, the insulating film is an integral element of the tube's operating parameters and such coatings degrade tube performance.
One method of alleviating the above noted problems includes providing surface conductivity to the insulating film. This includes covering the insulating film with a conductive coating. Due to its conductivity, the coating alleviates the accumulation of electrons and thus negative charges on the insulating film.
It is desirable that such a conductive coating be sufficiently thin so that the tube's performance is not substantially degraded. As is well known in the art, fabricating such a thin conductive coating with a uniform thickness is difficult to achieve with present manufacturing processes. Therefore, it is an object of the present invention to provide a conductive coating that is sufficiently thin so that a tube's performance is not degraded. In addition, it is an object of the present invention to provide a conductive coating that alleviates the accumulation of electrons on the insulating film of an image intensifier tube microchannel plate.